Molecular pathology is the examination at a molecular level of the DNA, mRNA, and proteins that cause or are otherwise associated with disease. From this examination important information about patient diagnosis, prognosis, and treatment options can be elucidated. The practice of molecular pathology is generally divided into two main areas: (i) analysis of DNA, mRNA, and proteins in intact cells (in-situ), and (ii) analysis of these biological materials after they have been extracted from tissues. The first category, to which the present invention is primarily directed, has the advantage that it allows the pathologist or scientist to study the histopathologic architecture or morphology of the tissue specimen under the microscope at the same time that the nucleic acid or proteins are being assayed. These techniques include immunohistochemistry (IHC) which looks at proteins, in-situ hybridization (ISH) which looks at nucleic acids, histochemistry (HC) which looks at carbohydrates, and enzyme histochemistry (EHC) which looks at enzyme chemistry. For example, ISH can be used to look for the presence of a genetic abnormality or condition such as amplification of cancer causing genes specifically in cells that, when viewed under a microscope, morphologically appear to be malignant. ISH is also useful in the diagnosis of infectious diseases as it allows detection not only of a microbial sequence but also of precisely which cells are infected. This may have important clinicopathologic implications and is an effective means to rule out the possibility that positive hybridization signal may have come from an adjacent tissue of no clinical concern or from blood or outside contamination.
IHC utilizes antibodies which bind specifically with unique epitopes present only in certain types of diseased cellular tissue. IHC requires a series of treatment steps conducted on a tissue section or cells (e.g. blood or bone marrow) mounted on a glass slide to highlight by selective staining certain morphological indicators of disease states. Typical steps include pretreatment of the tissue section to remove the paraffin and reduce non-specific binding, retrieval of antigens masked by cross-linking of the proteins from the chemical fixatives, antibody treatment and incubation, enzyme labeled secondary antibody treatment and incubation, substrate reaction with the enzyme to produce a fluorophore or chromophore highlighting areas of the tissue section having epitopes binding with the antibody, counterstaining, and the like. Most of these steps are separated by multiple rinse steps to remove unreacted residual reagent from the prior step. Incubations can be conducted at elevated temperatures, usually around 37.degree. C., and the tissue must be continuously protected from dehydration. ISH analysis, which relies upon the specific binding affinity of probes with unique or repetitive nucleotide sequences from the cells of tissue samples or bodily fluids, requires a similar series of process steps with many different reagents and is further complicated by varying temperature requirements.
In view of the large number of repetitive treatment steps needed for both IHC and ISH, automated systems have been introduced to reduce human labor and the costs and error rate associated therewith, and to introduce uniformity. Examples of automated systems that have been successfully employed include the NEXES.RTM. and Gen II.RTM. staining Systems available from Ventana Medical Systems (Tucson, Ariz.) as well as the system disclosed in U.S. Pat. No. 5,654,199 to Copeland et al. These systems employ a microprocessor controlled system including a revolving carousel supporting radially positioned slides. A stepper motor rotates the carousel placing each slide under one of a series of reagent dispensers positioned above the slides. Bar codes on the slides and reagent dispensers permits the computer controlled positioning of the dispensers and slides so that different reagent treatments can be performed for each of the various tissue samples by appropriate programming of the computer.
The aforementioned staining systems include either a hot air blower or a heat lamp to warm the samples above laboratory ambient temperatures for steps requiring elevated temperatures. Heating the slide improves staining quality by acceleration of the chemical reaction and can permit a reaction temperature more closely matching body temperature (about 37.degree. C.) at which antibodies are designed to react. While such convection or radiant heating systems have been generally suitable for IHC, which is antibody based, they are less suitable for ISH, which is nucleic acid based and requires higher and more precise temperature control. In order to denature the DNA double helix of both the target sample and the probe so as to render them single stranded, the temperature must be raised above the melting point of the duplex, usually about 94.degree. C. At the same time it is imperative that the sample not be overheated past 100.degree. C. as doing so destroys cell morphology making it difficult to view under a microscope. Precise temperature control is also required in ISH to effect probe hybridization at the desired stringency. The selected temperature must be low enough to enable hybridization between probe and template, but high enough to prevent mismatched hybrids from forming. It would be desirable, therefore, to have an automatic tissue staining apparatus which can control the temperature of reactions with enough precision for most ISH applications.
Another disadvantage of the heating units typically employed with automated tissue stainers is that they do not permit the temperature of individual slides to be separately controlled. With prior art systems all of the slides are heated to the same temperature at any given time during the process. For example, U.S. Pat. No. 5,645,114 to Bogen et al. discloses a dispensing assembly adapted to carry a plurality of microscope slides. Individual slide holders containing resistive heating units are provided. However, with the assembly taught by Bogen et al., all of the slides would be heated to a common temperature because, for example, no means are disclosed for separate heating controls or for shielding slides from heat generated by adjacent slides. This precludes protocols having different temperature parameters from being run on different samples at the same time. For example, DNA probe assays having different stringency requirements could not be run efficiently at the same time. It would be desirable, therefore, to have an automatic tissue staining apparatus wherein adjacent slides can have different tests applied to them even when the tests have unique heating requirements.
A difficulty frequently encountered in both IHC and ISH testing results from the manner in which the tissues are typically preserved. The mainstay of the diagnostic pathology laboratory has been for many decades the formalin-fixed, paraffin embedded block of tissue, sectioned and mounted upon glass slides. Fixation in such a preservative causes cross-linking of macromolecules, both amino acids and nucleic acids. These cross-linked components must be removed to allow access of the probe to the target nucleic acid and to allow the antibody to recognize the corresponding antigen. "Unmasking" the antigen and/or nucleic acid is typically accomplished manually with multiple pretreatment, protolytic digestion, and wash steps. It would be desirable if the process of conditioning cells so that their antigens and nucleic acids are available for detection could be automated.
Prior to staining, complete removal of the paraffin is also required so that it does not interfere with antibody or probe binding. Paraffin, a hydrophobic substance, must be removed prior to staining or hybridization using probes. Deparaffinization is normally achieved by the use of two or three successive clearing reagents that are paraffin solvents such as xylene, xylene substitutes or toluene which may be toxic, flammable and pose environmental hazards. Safer and faster methods to deparaffinize the slides would be advantageous.